1. Field of the Invention
The present invention relates to a multi-tap distribution apparatus, More particularly, the present invention relates to a multi-tap distribution apparatus which is suitable for use with CATV cables.
2. Description of the Related Art
In a CATV system, transmission cables are aerially strung on, for example, telephone poles or laid underground with a number of multi-tap distribution apparatuses and amplifiers interposed therein at suitable points. Secondary cables are branched out from multi-tap distribution apparatuses to subscribers' homes to distribute signals.
CATV stations transmit not only high-frequency signals, such as television signals and control signals, but also currents for operating amplifiers provided on the main lines and CATV telephones installed at subscribers' homes. Therefore, if a short circuit is caused by inappropriate installation of equipment or tampering by children, excess currents may flow through the cable to cause damage to the equipment installed in the cable.
In order to prevent such accidents, distribution apparatuses are normally provided with circuit-breakers that break the circuitry upon occurrence of a short circuit, One type of device used as a circuit-breaker is a positive thermistor (referred to as "thermistor" hereinafter), Thermistors are provided in series with respective taps (distribution output terminals) in the distribution apparatus' current transmission circuit for feeding the taps with operating currents. Any of the thermistors can interrupt the circuit upon occurrence of a short circuit as the resistance of the thermistor increases with the increase in the amperage of the current.
Where a CATV telephone system is used, each time a subscriber changes the CATV service company or cancels the subscription (which often occurs, especially in the United States), the power to the CATV telephone needs to be switched off and on. As described above, a positive thermistor or other suitable switch device can be installed in the current transmission circuit so that the circuit can be interrupted by operating the switch to stop the current supply to, and thus disable, the CATV telephone or other type of terminal.
FIG. 10 shows a conventional distribution apparatus used in a CATV system. This apparatus has an enclosure comprising a box-shaped housing 39 with one side open and a main body 38 that includes a plurality of distributing output terminals 5 and fits in the open side of the housing 39. The housing 39 is provided with transmission cable connection terminals 6 for connection to the main line. The apparatus contains a distributing circuit, a current transmission circuit, and a branch unit 7 having a branch circuit therein. Thermistors 14 are mounted as circuit breakers inside the apparatus via connectors 42 as illustrated, so that the distribution apparatus needs to be opened to remove or mount any of the thermistors 14.
Whenever the distribution apparatus needs to be opened to remove or mount the thermistors 14, the shield factor of the main line is reduced to allow external noise to enter the CATV line or to cause leakage of radio waves from the apparatus. Furthermore, since a plurality of subscribers are normally served from a single distribution apparatus, signal transmission to all of the subscribers is interrupted whenever the apparatus is opened. This inconveniences the subscribers as they cannot view the CATV programs or use the CATV telephone services even if only for a short time.